1. Field of the Invention
The invention relates to refrigeration systems, and more particularly to frost-free refrigeration systems for home use in which an automatically controlled defrost heater is provided incorporating a defrost heater.
2. Description of the Prior Art
Previously, the defrost cycle of a frost-free refrigerator was controlled by a timing motor connected directly across the compressor motor. Thus, whenever the compressor motor ran, the defrost motor ran, and after about eight (8) hours of compressor run-time, the defrost timer reached the defrost mode. The compressor was shut off, while maintaining power to the defrost timer. At this time the defrost heaters were energized as well as a condenser fan, and the frost build-up was melted and collected in the evaporating pan in the machinery compartment to be evaporated. The defrost heaters use a lot of energy, and this method of defrost cycle control is sub-optimal in situations where the doors of the refrigerator are not opened for extended periods of time, e.g., when the owner is on vacation or at night. Under these circumstances, no moisture enters the refrigerator, and the length of time between defrosts may be extended without the fear of excessive frost build-up. With the advent of microprocessor controlled appliances, it is possible to detect these situations and lengthen the defrost cycle appropriately. A shortcoming of the electronic control system was that the defrost system was not protected from catastrophic failure of the electronics; if the electronics failed in such a way as to disable power to the defrost timer, the defrost system would not cycle. Two modes of defrost failure can be identified. First, it is possible for the system to fail with the defrost heaters off, in which case a frost build-up would occur restricting the air flow between the freezer compartment and the fresh food compartment, causing an eventual overheating of the fresh food side. Even more detrimental is the mode in which the system fails with the defrost heaters on and the compressor disabled, in which case the refrigerator rapidly overheats. The bi-metal thermostat in series with the defrost heaters will interrupt the power, but the compressor will remain disabled.